The landing of an aircraft is a phase of the flight that is critical and complex. It requires the pilot to take account of diverse constraints, safety constraints firstly, but also economic constraints and comfort constraints.
First of all, a landing runway comprises several exit linkways spread the whole way along the runway. No exit is actually imposed on the pilot, who remains free to choose his exit depending on the situation which arises and notably depending on the climatic conditions. At best, he leaves the runway by taking one of the first of the exits to save time and to limit the consumption of kerosene on the ground. This additionally makes it possible to minimize the duration for which the runway is occupied. But optimizing runway exit is not an easy thing since numerous parameters come into play: state of the surface, meteorological conditions, weight and state of the craft, notably of the tires and braking system. This is why the choice of the runway exit linkway is not imposed, at the very most suggested by the controller. Finally, it is not enough to apply maximum braking to take the first exit, since this encourages premature wear of the brakes and heats the tires, penalizing the cost-effectiveness of the craft. Not to mention that exaggerated braking is always detrimental to the comfort of the passengers.
A purely manual solution consists initially, just after the main landing gear has touched the ground, in reversing the thrust of the jet engines. Then, subsequently, the pilot brakes by actuating the brake pedals acting on the wheels. The runway exit is chosen by guesswork by the pilot, who visually estimates the first exit that he can reach at a speed less than or equal to the maximum admissible speed for taking this exit. The maximum admissible speed for taking an exit is the speed above which taking the exit presents a risk having regard to the angle that the exit linkway makes with the runway. The maximum speed for taking an exit decreases as the angle increases. This angle may be as much as 90 degrees, this corresponding to a maximum exit speed of the order of 10 to 20 knots.
Another solution consists in the pilot being assisted by an automatic braking system called “auto-brake”, which makes it possible to select a deceleration level on an increasing scale varying from 1 to 2, from 1 to 3 or from 1 to 5 according to the airplane model. The system comes on immediately after the nose gear has touched the ground and brakes the airplane until it stops completely while complying with the deceleration level chosen by the pilot. The system is immutable and takes account neither of the particular landing conditions, such as the state of the runway or the meteorological conditions, nor of the speed of the airplane when it touches the ground. It does not guarantee any stopping distance, the latter is variable even for a given deceleration level. It is up to the pilot to compensate for the lack of flexibility of the “auto-brake” system by taking over when he estimates visually that he can take an exit. For this purpose he merely needs to actuate the brake pedals to deactivate the system. The result is then the same as for braking without the assistance of the “auto-brake” system: it is frequently necessary to open the throttle again in order to join an exit further on.
Whether in the case of a purely manual landing or in the case of a landing assisted by the “auto-brake” system, no means are currently available to the pilot enabling him to be certain in advance that the length of runway remaining ahead of the airplane is sufficient and that he will not overshoot the end of the runway. The availability of such information would allow the pilot to judge sufficiently in advance whether it is prudent to open the throttle again so as to take off again and attempt a new approach. Specifically, cases of missed landings are numerous where the pilot realized too late that he was going to overshoot the end of the runway, once down, no longer being able to open the throttle again and go around.
On 2 Aug. 2005 in Toronto, Canada, an airplane of Airbus A340 type with Air France flight number AF358 landed abnormally far along the runway. Despite the thrust reversers and the braking of the wheels, the airplane ended up in a ravine situated 200 meters after the end of the runway, which it crossed while its speed was still 150 kilometers an hour. Fortunately there were no victims, but the airplane was entirely destroyed. Not to mention the hardware cost, the loss is estimated at 75 million dollars.
In 2000 at Fredericton, again in Canada, an airliner of Fokker F-28 type on a Lignes Aériennes Canadiennes Régionales night flight left the end of the runway. This time, no human injury or hardware damage was suffered. The problem identified by the Canadian accident office mentions that the crew had nevertheless indeed been forewarned that the runway was 50% covered by a thin layer of melting snow. However, the decision to land was based mainly on the fact that the other 50% of the runway was only wet. But under such conditions, it is difficult to estimate the braking distance, a wet runway leading notably to aquaplaning during wheel braking. In all cases, the economic, and sometimes even human, consequences of such accidents can be catastrophic.
On the one hand, it appears that only a very small amount of information is currently available to the crew for estimating the risks incurred by landing on a contaminated runway, that is to say one whose surface is totally or partially covered by water, snow or ice. The crew uses braking performance tables which are provided in the technical documentation of the craft. They are available on board in a paper version or on-screen. With a runway state reported by the airport, of the good/average/bad braking type, and/or with a type of contaminant, these tables associate a theoretical braking distance calculated during in-flight trials. Then decisions are taken essentially on the basis of mental calculations and on the experience of the crew on this type of craft.
On the other hand, it is difficult to estimate the point at which the the wheels touch down on the runway, since this point varies with the type of approach and the wind conditions, notably with the tail wind. Now, the point where the airplane will touch the ground conditions the length of runway remaining for it to brake. The theoretical braking distance provided in the tables is calculated by assuming overflight of the runway threshold at 50 feet, the runway threshold being demarcated on the ground by wide paint bands parallel to the axis of the runway and forming a “comb”. But principally, the theoretical braking distance is calculated by assuming touchdown 300 meters after the runway threshold. Now, in difficult conditions involving wind, reduced visibility and/or a contaminated runway, it is almost impossible to ensure that these basic assumptions hold. Thus, the real touchdown point may be very far from the theoretical touchdown point 300 meters from the runway threshold. And once the airplane is going on the runway, it may be equally difficult to estimate whether the wheels touched down 300 meters or 600 meters from the runway threshold, or indeed further on still. The risks of overshooting the end of the runway are manifest.